1. Field of the Invention
The present invention relates to a process for the thermal treatment of a porous glass preform for the production of an optical fiber preform.
2. Description of the Related Art
A preform for an optical fiber is generally produced by forming a cylindrical or an annular porous preform by the Vapor Phase Axial Deposition method or the Outside Chemical Vapor Deposition method and heating, dehydrating and sintering the porous preform in a heating furnace under an atmosphere of an inert gas such as argon or helium, a chlorine based gas or a fluorine based gas to produce a transparent and highly pure preform for the optical fiber.
An operating temperature of the furnace depends on a kind of a dopant to be doped and a content of the dopant in the preform, and it is generally in the range of 1200.degree. to 1600.degree. C. Since impurities tend to contaminate in the preform at such high temperature, a muffle tube made of a high purity quartz is typically used in the furnace to prevent the contamination. However, the quartz made muffle tube tends to rapidly get brittle due to devitrification, that is, transition from a glass phase to a crystal phase at a high temperature, and thus it is poor in durability.
Recently, a carbon made muffle tube an inner surface of which is coated with silicon carbide (SiC) is often used in a heating furnace. The carbon made muffle tube can be operated at a higher temperature than the quartz made one and SiC improves a gas impermeability and oxidation resistance of the muffle tube.
Further, in order to prevent a reaction between SiC and a reactive gas, the SiC coating on the carbon made muffle tube is treated with oxygen to form a SiO.sub.2 layer on the SiC coating (see Japanese Patent Kokai Publication No. 201634/1986).
The prior arts as described above have following problems:
(1) When the quartz made muffle tube is used, it is softened and deformed at a temperature above 1400.degree. C. In addition, the temperature of the muffle tube cannot be lowered to a temperature below a crystal transition point (lower than 300+ C.) because the crystal (cristobalite) formation at a temperature above 1200.degree. C. Thus, once the muffle tube is heated, it should be used continuously without lowering the temperature thereof.
(2) When a SiC made or SiC coated muffle tube is used, SiC reacts with a halogen based gas as a reactant to form a porous carbon, whereby the muffle tube becomes poor in gas tightness so that the halogen based gas leaks outside from the muffle tube.
(3) In the case where the muffle tube having the SiO.sub.2 layer on the SiC coating is used, thermal expansion coefficients of SiC and SiO.sub.2 are greatly different from each other and thereby the SiO.sub.2 layer in a thickness in the order of some microns tends to crack. Then, a gas permeates through the crack and it is impossible to stably produce the preform for a long period.